It has been proposed to apply an ionic liquid owing to high ionic conductivity thereof to an electrochemical device as an electrolyte for a battery or an electrical double-layer capacitor. The ionic liquid has extremely high flame retardance, and hence when used as the electrolyte for the electrochemical device, there is no need for a combustible organic solvent, thus ensuring the electrochemical device with high safety.
The ionic liquid generally denotes salts having a melting point of 100° C. or less. The ionic liquids generally have higher melting points than an organic solvents, and some ionic liquids are frozen at room temperature, thus often failing to serve as the electrolyte for the electrochemical device. To allow the ionic liquid to serve as the electrolyte even at or below room temperature, a method has been proposed in which an ionic liquid is filled with or injected into nanopores of porous glass so as to decrease the melting point of the ionic liquid (patent document 1). A correlation between the size of nanopores and the melting point of the filled ionic liquid has also been reported (non-patent document 1). As a material with micropores having a smaller size than nanopores, the case of filled ionic liquid with a carbon nanotube has also been reported (non-patent document 2).
However, it is difficult to further decrease the pore diameter of the porous glass described in the patent document 1 than that of mesopores of approximately 75 Å. Therefore, when the ionic liquid is filled with the pores of the porous glass, the melting point of the ionic liquid is lowered by approximately 30° C. The term “mesopores” denotes pores having a diameter of 20 to 500 Å.
With the non-patent document 2, the ionic liquid is filled with the carbon nanotube with micropores having a further smaller pore diameter, that is, pores having a diameter of 2 nm or less. However, the carbon nanotube has electrical conductivity and hence cannot be used as the electrolyte for the electrochemical device.
According to the purpose of use, it may be desired to increase the melting point of the ionic liquid. With the non-patent document 2, the melting point of the ionic liquid is increased by filling the ionic liquid with the carbon nanotube. However, none of these documents describes the case where the ionic liquid is filled with a material having no electrical conductivity so as to increase the melting point.